1. Field of the Invention
The present invention relates to an improvement of a method of making semiconductor devices, and particularly concerns a method of forming an isolation region in a semiconductor wafer to define isolated island region(s) in an epitaxial layer formed on a substrate.
2. Prior Art
As is known, a bipolar type semiconductor integrated circuit (hereinafter referred as IC) is made by obtaining an island region of n-conductivity type in a n-type epitaxial layer grown on a p-type silicon substrate by forming a p.sup.+ -type isolation region to encircle and define the island region, and subsequently forming various kinds of devices and interconnections thereof.
In a high voltage type bipolar IC comprising transistors and diodes of high working voltage, the n-type epitaxial layer has such a large thickness of 15 to 50 .mu.m. Accordingly, the depth of the p.sup.+ -type isolation region, which must be extended from the surface of the n-type region into the underlying p-type substrate, must necessarily have a large depth of diffusion. And such a large depth leads to a long time of diffusion process, and hence a poor productivity. Furthermore when an n.sup.+ -type buried collector region of a high impurity concentration is formed in the p-type silicon substrate, the abovementioned long time diffusion process makes the high concentration n.sup.+ -type impurity into the n-type epitaxial layer, thereby lowering the working voltage of the device, which has been intended to be increased.
In order to shorten the diffusion time, a method of diffusing the impurity into the epitaxial layer from both sides thereof has been proposed. The method is that prior to the epitaxial growth, a first region of p.sup.+ -type impurity of a predetermined pattern is selectively formed on the substrate and a second region of p.sup.+ -type impurity of the same pattern is formed on the surface of the epitaxial layer, and then a diffusion is carried out to diffuse impurities of both the first and the second regions simultaneously, so that the impurities from both regions diffuse into the epitaxial layer and become connected by superimposing each other in the epitaxial layer. Hereinafter, let us refer the method as "graft isolation" method since the isolation region is formed by connection of the upward diffusion of the impurity of the first region and the downward diffusion of the impurity of the second region, and both the diffused regions are grafted each other. In the conventional graft isolation method, both of the first and the second region of p.sup.+ -type impurity are formed by disposing boron as impurity, and the surface impurity concentration of the p.sup.+ -type region is usually selected lower than 1.times.10.sup.19 atoms/cm.sup.3 in order to decrease adverse auto-doping of boron into the epitaxial layer. The problem of such conventional graft isolation method is that due to such limitation of the impurity concentration, and intended shortening of the diffusion time is not satisfactorily achieved, though the diffusion time is shortened to some extent in comparison with the old isolation method to diffuse only from the surface of the epitaxial layer. For example, when the abovementioned graft isolation method is used in forming an isolation region penetrating through an epitaxial layer of 30 .mu.m thickness and 10 .OMEGA.cm specific resistance by diffusing known boron from both sides of the epitaxial layer, the diffusion time under the temperature of 1200.degree. C. is so long as about 6 hours. As is abovementioned, a measure to increase the impurity concentration of the first and the second regions causes the adverse auto-doping, thereby deteriorating controllability of the specific resistance and crystal perfection of the epitaxial layer, and hence, the conventional grafting isolation method could not achieve a high productivity and long life of the device.